Electrophotographic process for the manufacture of a highly heat-resistant image



United, States Patent K 3,898 Int. Cl. G03g 13/04, 5/08 US. Cl. 96-1 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a photographic reproduction process which comprises the sequential steps of exposing an electrostatically charged photoconductive element to light under a master, said element comprising a layer of inorganic photoconductive pigment dispersed in an insulating resinous binder on a conductive support selected from the group consisting essentially of (l) a highly heat-resistant alloy having a nickel content in the range of 60-85 percent and a chromium content in the range of about 12-25 percent, and (2) steel having a nickel content in the range of 19-40 percent and a chromium content in the range ofv 18-26, developing the electrostatic image with a toner and fixing the resultant image, wiping the surface of the photoconductive insulating layer with a solvent therefor to remove nonimage areas thereof, and 'heating the layer at a temperature in the range of 1000 C. to 1300 C. for a time sufficient to sinter the toner image.

This invention relates to a process and a material for the manufacture of a highly heat-resistant image, using substantially scaling-resistant supporting materials made of special alloys, the supporting materials being coated with an electrophotographic layer containing an inorganic photoconductor on which an image is produced by electrop hotographic means, fixed, decoated and subjected to a heat treatment.

Electrophotographic materials comprising a support and a photoconductive layer are known, in which the support comprises metals, such as aluminum or copper, and the photoconductive layer consists of inorganic or organic substances. Materials of this kind yield a good image by the application of the customary electrop'hotographic methods, but their resistance to heat is limited, because in most cases the supporting material melts or decomposes at temperatures well below 800 C., or becomes otherwise unsuitable for reproduction purposes, e.g., by scaling, in addition to the fact that the electrophoiographically produced image runs or decomposes in all cases and thus becomes illegible.

There is, however, a definite demand in the technical and industrial field for the application of graphic information to highly heated parts, e.g., heavy-duty machine parts, devices used in the smelting of iron, and, above all, high-speed missiles used in aviation, which information must be clearly readable on the support or the part before, during, and after being subjected to very high temperatures.

Highly heat-resistant images and copies are of particular interest in view of their use as identification tags whose legibility must not be impaired by intensive heat action resulting from possible fuel fires in aircraft and automobile accidents, the eifects of modern warfare, and the like. 1

Of course, methods other than those of electrophotog raphy, e.g., engraving or embossing techniques, may

3,440,045 Patented Apr. 22, 1969 also be used for this purpose. However, these methods are very expensive as regards work and material required. Further, the copies thus produced do not show the details nearly as fine as is required in all cases.

The present invention provides a simple reproduction process for the production of a high precision copy which withstands temperatures up to at least 1200 C.

The present invention includes a process for the manufacture of a highly heat-resistant image by electrophotographic means, using an electrophotographic material comprising a support and a photoconductive layer comprising an inorganic photoconductor, which is charged, exposed, developed, fixed and decoated. In this process, a highly heat-resistant support is used and the image produced on this support is subjected to a heat treatment after being decoated.

The present invention also includes a material for performing the process according to the invention for the manufacture of a highly heat-resistant image by electrophotographic means, using an electrophotographic material comprising a support and a p-hotoconductive layer comprising an inorganic photoconductor. I11 this material, the support consists of highly heat-resistant alloys with a high nickel and chromium content.

The present invention further includes the product obtained by the process and material according to the invention, i.e., a highly heat-resistant image consisting of a highly heat-resistant support to which graphic information has been applied by sintering of a metal compound.

In the present invention, it has been found possible to produce a highly heat-resistant image by electrophotographic means and this may be achieved by electrophotographically producing on a highly heat-resistant support, which satisfies the requirements of electrophotography, in particular as regards the conductivity thereof, and carries a photoconductive layer, a fixed image which is decorated in the image-free areas. The image thus obtained is subjected to a subsequent heat treatment. This heat treatment must be of a duration and intensity such that, by the interaction of the support, the photoconductive layer still present in the image areas, and/or the toner covering the p'hotoconductive layer, a visible change in the surface of the support is produced which withstands temperatures up to at least 1200 C. As mentioned above, this change is produced by an appropriate selection of the supporting material and the after-heating step. The change may consist, e.g., in a differentiation of the physical or chemical properties of the areas of the support corresponding to the image areas, as compared with those areas Which may be called image-free areas, e.g., a differentiation in the scattering of light caused by superficial etching or dulling. Further,

such changes may be produced, e.g., by a chemical reaction of at least one compound contained in the photoconductor or toner layer with the supporting material, or by such a reaction of several compounds contained in the layer either with one another or with the support, or both, or by similar processes.

By means of the process and material of the present invention, an image is produced which is clearly legible before, during and after being heated, preferably up to 1200 C., but in particular cases also up to 1300 C. or even 1400 C. The reproduction of the image areas is sharp and rich in contrast. Further, the methods for the production of such an image are simple and can be performed by the usual devices. Thus, clearly legible graphic information can be applied to machines and workpieces which are heated to high temperatures. The present invention is particularly suitable for the manufacture of highly heat-resistant identification tags which permit the identification of bearers thereof even after accidents caused by ignited fuel or the action of atomic warfare, and the like.

Supports to be used in accordance with the present invention are materials which are highly heat-resistant, i.e., they must exhibit only little scaling effect at the temperatures involved and must be highly resistant to the influence of air and hydrogen. Moreover, plastic deformation thereof must begin only at temperatures in excess of 1200 C. to 1350 C. Preferred materials of this type are, e.g., special steels and nickel-chromium alloys.

Steels fulfilling these requirements are, e.g., those which have a high nickel and chromum content. High content in this connection means a content of about 19 to 40 percent, preferably 20 to 37 percent, of nickel, and a content of 18 to 26 percent, preferably 20 to 25 percent, of chromium. In special cases, when the content of nickel is very high, a chromium content of about 16 percent will be suflicient. Minor percentages of silicon, aluminum, molybdenum, or titanium may be added to these steels in order to improve other physical or chemical properties. Minor percentages in this connection are about 0.1 to 4 percent, preferably 0.1 to 2 percent. The carbon content is in general below 1 percent.

Steels which satisfy these conditions very well are the so-called Thermax steels sold by Deutsche Edelstahlwerke A.G., Krefeld, Germany. Of these steels and of steels produced by other firms, the following may be used with particularly good results:

TABLE Content. in percent by weight Cr Ni Si O Thermax Steel, type 11A (Deutsche Edelstahlwerke A.G., Krefeld) 25.0 Thermax Steel, type 16/36 (Deutsche Edelstahlwerke A.G., Kreield) 16.0 Marwedur type A 63 (Mannesmann)- 25.0 Nimonic type DS (Wiggin Co. Ltd.,

Birmingham, Great Britain) 18.0

Nickel-chromium alloys in the sense of the present invention are those which have about the following contents:

Alloys which are particularly suitable for the purposes of the present invention are the following (the percentages stated are by weight):

Ni Cr Co Fe Ti, Nb. Alor others Nimonic type N 80 (Wiggin Co. td., Birmingham, Great Britain) 0 0 Nimonie type N 90 (Wiggin Co. Ltd., Blrming am, Great Britain) Inconel (Inco, U.S.A.).

Inconel type X (11100, U .A.)

6.5 Some percent. 5-9 Some percent.

Steels and alloys of the aforementioned type have a very high melting point, bewteen about 1300 and 1500 C. Moreover, they possess a pronounced scaling resistance at such temperatures, even in an oxidizing atmosphere. In connection with the present case, a material is termed scaling resistant when the average weight of metal lost by sealing does not exceed 1 g./m. per hour at a given temperature, or 2 g./m. per hour at a temperature 50 C. higher, during an overall heating time of 120 hours, interrupted by four intermediate cooling periods. The materials described above may be used, e.g., in the form of thin laminae or as foils which may be either continuous or cut to size.

In the invention, the aforementioned supports are coated with a photoconductive layer. This layer comprises one or more photoconductive substances of inorganic nature, to which, if desired, organic photoconductors and/ or binders and/ or sensitizers and/ or substances which improve the adhesion to the support may be added, even in large quantities.

Suitable photo-semiconductors for the photoelectrically conductive insulating layers of the present invention are, above all, the customary inorganic compounds, e.g., the sulfides and oxides of cadmium and zinc. Zinc oxide is particularly suitable for the process of the present invention. Mixtures of these inorganic photo-semiconductors with one another or with organic photo-semiconductors also may be used with good results.

Organic photo-semiconductors which may be used for the purposes of the present invention are: oxadiazoles, such as 2,5-bis-(4-(n-propylamino)-2-chlorophenyl-( 1) l,3,4-oxadiazole; imidazolones, such as 4,5-bis-(4-aminophenyl)-imidazolone-(2); imidazolethiones, such as 4,5- diphenylimidazolethione-(2); triazoles, such as l-methyl- 2,5 bis-(4-N,N-diethylaminophenyl-(1'))-1,3,4-triazole; oxazoles, such as 2-(4'-chlorophenyl) phenanthreno- 9',10:4,5 oxazole; thiazoles, such as 2-(4'-dimethylaminophenyl)-6-methoxybenzthiazole; triazines, such as 3-(4'-aminophenyl)-5,6-di-pyridil-(2")-1,2,4-triazine; and hydrazones, such as anthracene-9-aldehyde-phenylacetic acid-hydrazone.

Since the photoconductors do not adhere very well to the supports in most cases, it has been found advantageous to use them in admixture with binders. The following binders are preferred: natural and synthetic resins, e.g., balsam resins, colophony-modified phenol resins, and other resins with a high content of colophony, coumarone resins and indene resins, and the substances included in the comprehensive term lacquer resins which include, according to the Kunststoiftaschenbuch (Plastics Pocket Book) by Saechtling-Zebrowski (11th edition, 1955, pages 212 et seq.): modified natural substances, such as cellulose ethers; polymers such as the polyvinyl chlorides; polyvinyl acetate, polyvinyl acetals, polyvinyl alcohols, polyvinyl ether, polyacrylic ester and polymethacrylic ester, also polystyrene and isobutylene, polycondensates, such as polyesters, viz phthalate resins, alkyd resins, maleinate resins, maleic acid/colophony mixed esters of higher alcohols, phenol-formaldehyde resins, in particular colophony-modified phenol-formaldehyde condensates, urea-formaldehyde resins, melamine-formaldehyde condensates, aldehyde resins, ketone resins, also xyleneformaldehyde resins and polyamides, and polyadducts, such as polyurethanes.

Of particular advantage for the purposes of the present invention is polyvinyl acetate into which acid groups, or groups which are easily converted into acid groups, are introduced by polymerization. A polymerization product of vinyl compounds with crotonic acid, maleic acid or maleic anhydride is particularly advantageous. If acid binders are used, it is advisable to add amines, preferably volatile amines.

The ratio of photoconductor to binder may vary within wide limits. Mixtures of 2 parts of binder to 1 part of photoconductor substance to mixtures of 10 parts of photoconductor to 1 part of binder are preferred. Photoconductor/binder mixtures of ratios by weight from 5:1 to 2:1 have proved to be particularly advantageous.

For improving the light-sensitivity of the electrophotographic insulating layer in the range of visible light, small quantities of sensitizers, i.e., from about 0.0001 to about 0.1 percent by weight calculated on the quantity of photoconductor present, may be added either to the sensitizing solutions or to the finished electrophotographic material by an after-treatment. Such sensitizers, which for the most part are dyestufis, are disclosed, e.g., in Belgian Patent No. 558,078. If zinc oxide is used as the photoconductive su bstance, anionic dyestuffs yield best results.

Small amounts, i.e., a few percent, of other substances, such as pigments, fillers or activators, also may be included in the photoelectrically conductive insulating layer.

Suitable additives which improve the adhesion of the image areas to the supports are, above all, such compounds which, when heated, create a condition of the support which increases the aflinity thereof to the image areas, e.g., inorganic or organic salts of hydrofluoric acid, such as sodium fluoride, potassium fluoride, and calcium fluoride, and of phosphoric acid. Alternatively, these substances may be applied in the form of an intermediate layer between the support and the photoconductive layer.

Besides improving the adhesion to the support, these substances have, in general, the added advantage that they increase the flowing capacity of the substances used for distinguishing the image areas.

Toners for use in the present invention are the inorganic and/or organic substances customarily employed in electrophotographic processes. Exemplary of suitable inorganic toners are: finely powdered inorganic substances, such as salts, e.g., potassium sulfate, calcium sulfate, ammonium chloride, sodium chloride, potassium bromide, copper sulfate, aluminum/potassium sulfate, and sodium sulfate; oxides, e.g., iron oxide, titanium dioxide, zinc oxide, aluminum oxide, and copper oxide; silicates, e.g., kieselguhr, silica gel, talcum, and glass powder; borates, e.g., sodium metaborate, and potassium borate; and carbonates, e.g., calcium carbonate, magnesium carbonate, and potassium carbonate.

The following substances are particularly suitable as organic toners: natural and synthetic resins, such as colophony, copals, dammar resin, asphalts, colophonymodified phenol resins, ketone resins, maleic resin, coumarone resin, polyacrylic acid resin and polystyrenes. Mixtures of these resins also may be used. Inorganic and/or organic pigments and/or dyestuffs are added to these resins. Suitable substances are, e.g.: carbon black, zinc oxide, titanium dioxide, barium sulfate, minium, and the following dyestuffs which are taken from Schultz Farbstoiftabellen, vol. 1, 7th edition (1931); Heliorot RMT (No. 200), Helioechtblau (No. 1188), Sudan dyestuffs, e.g., I '(No. '33), II (No. 92), III (No. 532), CB (No. 127), R *(No. @149), Cellitone dyestuffs (Color Index, vol. I, 2nd edition, 1956, pages 1655-1742), 'Nigrosin spirit-soluble '(No. 985), Pigmentschwarz B -(No. 1361), Alizarinblauschwarz B (No. 1195), Diamantschwarz F (No. 614), Fanalviolett LR (No. 803), and mixtures of such dyestuffs and/or pigments.

Further, it is often advisable to add to the toners small amounts, e.g., 0.1 to 10 percent by weight, of waxes and/ or organic substances with low melting points, or the substitution products thereof, in order to favorably influence the melting point and adhesion of the toner mixture.

Suitable waxes for this purpose are: natural waxes, such as carnauba wax, beeswax, Japan wax, montan wax and ceresin, and synthetic waxes, such as the products commercially available under the designations A-wax, OP-wax, SPO-wax, V-Wax, O-wax, 'E-wax, hard wax H, hard wax W, several other so-called 'Ruhr waxes, and particularly the products sold under the name Gersthofen waxes and marked S, L, 0, OP, also wax-like substances, such as hard paraffin, stearic acid, high-pressure hydrogenated waxes, and stearic alcohol. Exemplary of organic compounds with low melting points are in particular substituted and unsubstituted aromatic compounds which melt between about 40 and 150 C.

Compounds of this type are naphthols, such as l-naphthol and Z-naphtholjalso aromatic compounds, such as acenaphthene; acylamino compounds, such as acetanilide; halogenated aromatic compounds, such as p-dibromobenzene; amino compounds, such as 2,4-diamino toluene and o-phenylene diamine; phenols, such as resorcinol; and diphenylamine and its derivatives.

The toners are prepared by very finely grinding a mixture of the preferably comminuted starting materials, heating the mixture to the melting point, agitating the melt until it is substantially homogeneous, and then cooling it. Alternatively, the mel-table starting materials may be liquefied by heating and the other components introduced with agitation, after which the mass is cooled. The toner mass thus obtained is milled and screened. Screened fractions of an average grain size of about 0.1 to about 50 1., preferably of about 1 to 10 and/or about 10 to 20 4, are used for the toner.

A suitable developer also may be produced in known manner by dispersing the aforementioned substances in a dielectric liquid, such as aliphatic hydrocarbons and fluorinated aliphatic hydrocarbons.

Images are produced electrophotographically as follows: after the photoconductive layer has been charged, e.g., by means of a corona discharge from a charging device maintained at 6000 to 7000 volts, the support, e.g., a highly heat-resistant steel plate, with the sensitized layer thereon is exposed either under an original or by episcopic or diascopic projection, and then dusted over in known manner with a resin powder colored with carbon black. The image which thus becomes visible may be easily wiped away. It is therefore fixed, either by a brief heating to about C., or, depending upon the melting temperature of the developer used, by means of an infrared radiator. The temperature may be decreased when the heat treatment is elfected in the presence of solvent vapors, such as those of trichloroethylene, carbon tetrachloride, or ethyl alcohol. Fixing also may be effected by a steam treatment. lositive images distinguished by good contrast effects are obtained from positive masters. I

The electrophotographic images thus obtained are decoated after fixing, i.e., the surface of the photoconductive layer is wiped over with a solvent for the layer, e.g., alcohol, acetic acid, or preferably an alkaline medium. The solvent is selected with regard to the chemical nature of the binder used. By this decoating step, the image-free areas are removed, thus baring the metal support.

The deco ated image is then subjected to a heat treatment, i.e. the electrophotographic material bearing the image is heated overall. The temperature attained during the heat treatment will generally be over 1000 C., preferably between 1150 and 1300 C. In particular cases, lower temperatures will be sufficient, or higher temperatures will be advantageous in order to bring about the desired results of the invention. The heat treatment need not be long. In most cases, it will be sufiicient to expose the electr-ophot-ographic material to the required temperatures for a period of a few minutes, e.g., up to '5 minutes. The preferred times of exposure to heat are in the range of about 1 to 10 minutes. This heat treatment may be performed, e.g., in a muffle furnace, or by means of another device or system which produces the quantity of heat required.

The invention will be further illustrated by reference to the following specific examples:

EXAMPLE 1 400 g. of zinc oxide and 200 mg. of Bengal rose are ground for 4 hours in a ball mill with 1600 ml. of ethyl alcohol. A solution of 200 g. of polyvinyl acetate with a content of crotonic acid of 5 percent (Mowilith CT 5) in a mixture of 600 ml. of ethyl acetate and 20 ml. of morpholine is added to this dispersion. The dispersion is thoroughly agitated and then applied by means of a spray gun to a highly heat-resistance workpiece to be provided with an image. The area of the workpiece coated with zinc oxide is then negatively charged in known manner by means of a corona device in the absence of actinic light. The writing or drawing to be reproduced is then projected onto the charged layer.

The charge pattern thus produced, which corresponds to the projected image, is then made visible by means of a developer powder obtained by grinding and sifting a melt of 30 g. of polystyrene, 30 g. of maleinate resin,

and 3 g. of carbon black. This developer powder is advantageously mixed with iron filings, and the mixture is applied by means of a magnetic brush. The powder image thus produced is then fixed by a heat treatment of about minutes duration, the temperature being 180 C.

The area of the workpiece coated with zinc oxide and then provided with an image is now treated with a solution containing 92.5 parts by weight of distilled water, 7.5 parts by weight of isopropanol, 2.0 parts by weight of sodium metasilicate, and 0.5 part by weight of hydroxyethyl cellulose, until the zinc oxide layer is removed from the image-free areas. A black image on a metallic grey background appears.

This image is again subjected to a heat treatment. For this purpose, the workpiece is heated for about 3 minutes to 1200 C., e.g. in a mufile furnace. A weakly yellowcolored, sharp image with good contrast becomes visible on a greyish-black metallic background. This image withstands further action of high temperatures, is easily readable, and resistant to wiping when cooled.

EXAMPLE 2 By the procedure described in Example 1, a highly heat-resistant, 0.8 mm. thick steel plate, e.g. Thermax 11A, a product of Deutsche Edelstahlwerke A.G., Krefeld, Germany, is coated with a zinc oxide dispersion. The coated plate is then electrostatically charged, exposed in contact with an original having writing thereon, developed with a developer powder, fixed and decoated, as described in Example 1. A black image is obtained on a grey background.

The Thermax steel plate with the writing thereon is now heated for 3 minutes in a muffle furnace at a temperature of 1200 C. After cooling, a light-yellow image on a greyish-black background is obtained which is resistant to the further action of high temperatures. Individual labels may be punched out of this steel plate and used as identification tags. If desired, the tags bearing the graphic information may be coated with a protective lacquer. Before applying the lacquer, they may be backed with a second metal plate so that an interior space is formed which accommodates additional data or a radiation dosimeter.

It will be obvious to those skilled in the art that many 4 modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is: 1. A photographic reproduction process which comprises the sequential steps of exposing an electrostatically charged photoconductive element to light under a master, said element comprising a layer of inorganic photoconductive pigment dispersed in an insulating resinous binder on a conductive support selected from the group consisting essentially of (1) a highly heat-resistant alloy having a nickel content in the range of -85 percent and a chromium content in the range of about 12-25 percent, and (2) steel having a nickel content in the range of 19-40 percent and a chromium content in the range of 18-26, developing the electrostatic image with a toner and fixing the resultant image, wiping the surface of the photoconductive insulating layer with a solvent therefor to remove non-image areas thereof, and heating the layer at a temperature in the range of 1000 C. to 1300 C. for a time sufiicient to sinter the toner image.

2. A photographic reproduction process according to claim 1 in which there is an intermediate layer between the support layer and the photoconductive insulating layer, the intermediate layer comprising at least one compound which improves the adhesion of the insulating layer to the support layer and which compound is selected from the group consisting of sodium, potassium, or calcium salts of hydrofluoric or phosphoric acids.

References Cited UNITED STATES PATENTS 1,718,945 7/ 1929 Carter 96-34 1,735,744 11/1929 Fry 171 2,190,781 2/ 1940 Guertler 75-171 2,681,473 6/1954 Carlson 961 X 3,008,825 11/ 1961 Van Dorn et al. 961.'8 3,121,006 1/1964 Middleton et a1 961.5 3,172,828 3/1965 Shepard et al 96-1.5 X 3,352,669 11/1967 Murphy 96-15 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

US. Cl. X.R. 

